In the field of added value telephone services, there is an increasing demand for transmission of video signals and data in addition to the transmission of audio signals. In particular, the need to exchange the above mentioned signals both in local environments (e.g. users of a same company) and remote environment (e.g. users of two different companies placed in distant geographical areas even hundreds of kilometers apart) is now well recognized.
In order to meet the above mentioned requirements, users must be supplied with a working station to facilitate teleconferencing or the like including equipment such as:
one telephone set; PA1 one terminal equipped with display; PA1 one telecamera. PA1 coupling means connected to at least one line suitable to transmit/receive video signals towards the remaining switching nodes, as well as one working station, at least through a video-phonic connection line; PA1 control means, receiving signalling messages generated at at least on work station, to generate signalling messages relevant to the state of said at least one video line, and to drive said coupling means; PA1 means for switching of telephonic signals to which the telephone sets of each working station are connected through telephonic lines; PA1 means for data transmission/reception, associated to each working station, suitable to send, or receive, to or from, the working station of the calling user through said means for phonic signal switching signalling messages received from, or to send to, the control means of the relevant switching node.
These work stations are connected to a switching node which must also be connected to one or more remote switching nodes by means of transmission lines suitable for the transmission of video signals, possibly in a compressed form. Of course the connection between two users abutted to two different switching nodes involves the presence, in each node, of means suitable to detect signalling criteria and also to send them towards the remote switching node.
In a known solution, the unit controlling a switching node is configured in such a way to detect signalling criteria relating to the video link and to send the signalling criteria, through a signalling channel abutted the video link, to the control unit of the remote switching node. This solution allows the connection between two geographically distant users, but is expensive since the control unit of each switching node is of a complexity which can be compared to that of control units of a digit switching exchange.
In the field of wide band communication systems, the problem of diffused services must be considered, that is the problem of sending on whichever matrix output is available, a predetermined number of channels which can in this way be distributed to users requiring the same. Audio, visual and data sources can be allocated to diffusion channels which can be distributed to selected users connected to the switching matrix which are interested in receiving signals emitted by such sources. This is the case for instance of a lesson recorded on videocassette and transmitted on a selected diffusion channel to be received through the switching matrix by all concerned users.
Solutions are known foreseeing the use of a "monostage" matrix, that is a matrix having a number of crossing points equal tot he number of inputs for each of the number of outputs. These solutions allow the immediate realization of diffusion services but are costly, especially when the number of inputs/outputs is high (e.g. 200 inputs and 200 outputs). Currently, the cost of a crossing point of the subject type is approximately 40 dollars and therefore the cost of the mentioned matrix totals to 1.6 million dollars.
Solutions exist which reduce the number of crossing points but foresee the realization of multistage matrixes having a higher blocking probability which increase with increasing reduction of crossing points compared to the above mentioned number.
Solutions also exist which allow a reduction in the number of crossing points without increasing the matrix blocking probability (they are known in the specific technique referred to as "closs"), but these solutions do not allow the realization of diffusion services.
A generic working station, as specified above, can be connected to a working station abutted to a remote switching node by means of coding equipment suitable to compress the band of the video signal.
Such coding equipment is configured to emit, at the output, bit strings obtained by processing the video signal received at the input. This processing operations require a determined time, which is sometimes of variable magnitude with he result that the remote user receives the audio signal and, with a variable delay, the video signal. The resultant effect is disturbing, for example, when the face of the remote user is framed, since the voice is received in advance of the image reproducing movements of the mouth. To overcome this problem, solutions have been identified which insert delay circuits into the audio lines to introduce a delay equal to the one introduced by coding equipment on video signals.
Since the coding equipment introduce a variable delay related to the speed of change of images shot by the telecamera, the delay circuits of the audio signal are expensive and complicated to realize.